Offshore freshened groundwater in the Pearl River estuary and shelf as a significant water resource

Large-river deltaic estuaries and adjacent continental shelves have experienced multiple phases of transgressions and regressions to form interlayered aquifer-aquitard systems and are expected to host vast paleo-terrestrial groundwater hundreds of kilometres offshore. Here, we used offshore hydrogeology, marine geophysical reflections, porewater geochemistry, and paleo-hydrogeological models, and identified a previously unknown offshore freshened groundwater body with a static volume up to 575.6 ± 44.9 km3 in the Pearl River Estuary and adjacent continental shelf, with the freshwater extending as far as 55 km offshore. An integrated analysis of stable isotopic compositions and water quality indices reveals the meteoric origins of such freshened groundwater and its significance as potential potable water or raw water source for desalination. Hotspots of offshore freshened groundwater in large-river deltaic estuaries and adjacent continental shelves, likely a global phenomenon, have a great potential for exploitable water resources in highly urbanized coastal areas suffering from freshwater shortage.

Text S1. Schematic diagram of a Rhizon sampler extracting porewater from offshore holes and measured salinity profiles.
Generally, a Rhizon sampler consists of four parts as mentioned in methods section of the manuscript. The thin, porous tube is inserted directly into an intact sediment core, and a 20 mL syringe is attached to the connector. The vacuum in the syringe is the main driving force for extraction of the porewater from the sediment core. Porewater then passes from the sediment through the porous tube and flexible hose into the collection syringe (20 mL). Furthermore, a threeway valve is added between the connector and syringe to facilitate multiple samplings in one position ( Figure S1a). Given a sufficiently small tube pore size (0.15 µm), the Rhizon sampler also serves as a filter, removing microbial and colloidal particles. Before the anterior tube (with a hydrophilic membrane) is inserted into the core, a portable drill is used to make a small hole (5 mm) in the core barrel.    Note: SW, seawater. DW, drinking water, the standard mainly refers to Ministry of Health of China, but the limitation of heavy metal Sr refers to the U.S. EPA. The concentration of NOxis counted by "N". n.a. means the concentration of the index is below 5.0 ppb or the detection threshold of the analytical method. Other heavy metals, i.e., Be, Cd, Cr, Co, Ga, Pb, Se, Te, Tl, and Bi with concentrations below this detection threshold are not listed.  .0 9518.7 339.9 1026.1 305.8 15471.4 2030.2 0.6 29.4 2299.5 n.a.  n.a.  60.8  n.a. n.a. 4041.1 n.a.

Text S2. The OFG caused by decomposition of gas hydrates and dehydration of clay minerals in the South China Sea
In the northern continental shelf of the South China Sea, two other emplacement mechanisms can also contribute to a low-salinity anomalies in seabed porewater. Postsedimentary alteration processes can lead to the release of fresh water and the formation of freshened water fronts in deeply buried marine sediments or in high pressure convergent margins ( Figure S3a  Buried paleochannels show a typical characteristic of discontinuities and "V" or "U" downcutting in the sub-bottom seismic reflections.
However, the Quaternary strata is delineated in the seismic reflections by continuous, high-amplitude and mid-strong reflections and labelled T20, which can be continuously tracked in the study area 1 . Some typical profiles can be found in previous studies 1, 2, 3, 4, 5 .

Text S4. Reconstruction of sediments basement in the PRE and adjacent shelf.
The seismic-reflection survey to characterize the offshore faults and basement in the PRE and adjacent shelf was conducted by research vessel "Haidiao-6" in May 2015. A total of 13 twodimensional, 24-channel seismic-reflection profiles were obtained, which included 10 lines in the northwest-to-southeast direction (S1-S8) with lengths between 115 and 145 km and three northeast-to-southwest trending profiles (W1-W3) with lengths exceeding 210 km ( Figure S5). The

Text S5. Geostatistical model and paleo-hydrogeologic modelling settings.
In the offshore domain, the upper model cells and the rightmost column are assigned a specified head boundary, which equals the sea level elevation according to the reconstructed eustatic sealevel curve, with a concentration of 35 g L -1 . For nodes above sea level, we imposed a specified flux along the top boundary to simulate the groundwater recharge. As no long-term precipitation record exist for the Pearl River delta over the past 125 kyr, we chose to apply a constant uniform recharge of 1.23 mm d -1 , which is equal to about 25 % of the current long-term precipitation average close to similar previous study 8 . The variable-density groundwater flow and coupled salt transport modelling software SEAWAT is used to solve the simulations using the finite difference method.   Figure S7. Porosity obtained from unconsolidated sediments in offshore boreholes from previous studies 2, 9,10 . (a) The locations of the offshore boreholes in the PRE and adjacent continental shelf, (b) Porosity versus depth of the unconsolidated sediments obtained from these offshore boreholes.